Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

[Hannes Frederic Sowa]

On Wed, Oct 09, 2013 at 05:28:33PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 09, 2013 at 05:12:40PM -0700, Eric Dumazet wrote:
> > On Wed, 2013-10-09 at 16:40 -0700, Josh Triplett wrote:
> > 
> > > that.  Constructs like list_del_rcu are much clearer, and not
> > > open-coded.  Open-coding synchronization code is almost always a Bad
> > > Idea.
> > 
> > OK, so you think there is synchronization code.
> > 
> > I will shut up then, no need to waste time.
> 
> As you said earlier, we should at least get rid of the memory barrier
> as long as we are changing the code.

Interesting thread!

Sorry to chime in and asking a question:

Why do we need an ACCESS_ONCE here if rcu_assign_pointer can do without one?
In other words I wonder why rcu_assign_pointer is not a static inline function
to use the sequence point in argument evaluation (if I remember correctly this
also holds for inline functions) to not allow something like this:

E.g. we want to publish which lock to take first to prevent an ABBA problem
(extreme example):

rcu_assign_pointer(lockptr, min(lptr1, lptr2));

Couldn't a compiler spill the lockptr memory location as a temporary buffer
if the compiler is under register pressure? (yes, this seems unlikely if we
flushed out most registers to memory because of the barrier, but still... ;) )

This seems to be also the case if we publish a multi-dereferencing pointers
e.g. ptr->ptr->ptr.

Thanks,

  Hannes

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